I was talking to a friend who is playing the game and is about level 10 so still not too deep into it yet. He has encountered the "House of Reardon" quest just now and was telling me of his encounter with another witcher named Letho within that quest, and how he went on to help the guy yada yada.

Now the weird thing is... I've completed the entire game and I also did the House of Reardon questline - but I've never encountered Letho anywhere period, nor have I encountered any booby traps in the barn, the ladder you're supposed to use to get to where he is, etc. I only know of these things from discussion with my friend and some youtube videos. So I'm wondering why this was not present in my game and if there are choices that you make in the game which might decide whether or not you encounter Letho?

I know Letho is allegedly an important character from the previous game (although I haven't played it yet). One might be tempted to think that perhaps he only shows up if you started The Witcher 3 using a save game from The Witcher 2 and that this unlocks him as an easter egg or something, but neither me nor my friend have played the 2nd game nor had save games to start Witcher 3 off so that's not possible. Nonetheless, he encountered Letho and I have not, and I kind of feel I missed out on a bit of an exciting questline.

I am at the House of Reardon in my after-game right now looking around for anything that might trigger it but... nothing. ;o/

Any ideas not covered above?

Ok, I'm back in the barn now and found something carved into the wall with my witcher senses. When examined, Geralt says "Delores" then it registers as me having seen it, and no longer can be examined, but in it's place slightly to the left and down is now "[E] Locked" but there's nothing there. It appears as if it might be on the other side of the wall, but the other side of the wall is a staircase leading into a basement under the garage which I explored fully long ago and found hidden stuff etc. which is all taken care of. So there is this locked thing there that does not appear to be accessible.

Your friend probably used the save simulate option when starting a new game. You get asked questions about the events of the last game when you're getting shaved before seeing the emperor, and he probably did that and chose that he let Letho go.

"Allegedly important"... Lol. You sir or madame, have a gift for understatement.

Not sure what that is. I did some googling though and found out that indeed your choices in The Witcher 2 can affect whether or not you see Letho in Witcher 3, and if you haven't played 2, then some questions you're asked in 3 will set the "witcher 2 game state" so that's probably what you're mentioning. I probably chose the wrong options unknowing either way what the outcome would be, and probably locked myself out of that questline possibility. ;o/

Nonetheless I do find Delores name on the wall in the barn and on the wall beside it "[E] locked" which may or may not be related to the Letho questline, but it is annoying me that I can't find out anything about what it is or how to get into it, and I'm completely done that questline and Delores doesn't talk to me anymore other than to thank me for my help. Hopefully I will trigger the Letho questline in my next playthrough though. The word selection is based on my personal knowledge and mind-state of The Witcher universe. I have no personal knowledge of who the character Letho is and as a result of that no idea how important he may or may not be, nor have I seen any videos or other official materials of who he is, nor official references about him. My only knowledge of who he is are from a vague reference to him in another forum post and a short discussion with a friend who did unlock the quest who has no knowledge of the prior Witcher games or universe either.

I have no reason to believe he is not important but I've not seen any evidence first hand to suggest he is either, but I do have reason to believe he might be important based on a very small bit of second hand information. Therefore, I allege he is important, using the following definition:

http://dictionary.reference.com/browse/allege?s=t

----
verb (used with object), alleged, alleging.
1.
to assert without proof.
----

That is to say that I myself assert without any first-hand proof that he is important. Someone else with advance knowledge of who he is and has seen the proof such as by playing the game or watching a video of it or something need not allege anything if they have witnessed direct evidence.

Sadly, I have not - so I allege, for he could be a town beggar that scrapes gum off of people's boots for fisstech for all I know. :)

Wow.

Well, anyway, about your continued confusion over Letho: When you start a new game you'll have an option to "Simulate Witcher 2 Save File" or something like that. Select yes and you'll get the option to tell someone about Geralt's choices in Witcher 2, and thus whether or not Letho is a factor.

I just noticed that I imported the wrong save from TW2 because TW3 put the saves in alphabetical order instead on chronological one (from the most recent).

I played last year or so, so I imported a save at the start of the 3rd act or so. :(

Seriously the sorting system should be totally revised in all the part of the game. :(

Same thing happened to me; I was over 30 hours in when I discovered it and had cleared everything south of Novigrad and was damned if I was going to do it again. Maybe if I play through the whole thing again. Might happen if and when they release the mod tools.

You can do that quest anyway, but it will be different if there isn't Letho. I killed Letho in The Witcher 2 and I've found Reardon manor full of wraiths instead of traps.

Isn't the scene where the nilfgardian general interrogate you (while the guy is shaving you) supposed to be used to set this kind of variable?

Yes it is, but only if you haven't imported a Witcher 2 savegame, which I did, and I didn't play that scene.

You also have to select "simulate a Witcher 2 save" (or something to that effect) when starting a new game. That shaving scene isn't an automatic thing when you don't import a save. You have to actually select it.

EDIT: Or, to be more clear, the shaving itself always happens, but you only get interrogated if you choose the "simulate" option.

I see, I thought it was standard for everyone who didn't import a save.

Ah, that's what that meant. Would have been nice if they had something more descriptive to indicate what the various options did as I was a bit confused by that, as were about 3 other friends that went to start a game with no prior experience with The Witcher games. :) Good to know what it means though, thanks.

Yeah, I totally plan on playing TW2 for sure once I complete TW1 at least once. :)

I just went back there and very very carefully examined the inside of each building in every nook and cranny and couldn't find anything. The building that Delores shows up in has a small box on the floor in the one corner which flashed for a fraction of a second "[E] <something>" although it was very brief and I didn't see what it said, but no matter how I walk/dance around it it never comes back so I presume it's a minor game display glitch.

Does the key only become available if the Letho subplot is completed perhaps? Yeah, when I went there the first time there were wraiths too and no Letho so it must have been due to the way I started the game plus dialogue choices as people suggested. Guess there was no way to know in advance though. :)