I made one particular character have very high Chr and very high Diplomacy...

But after a conversation or screen change the game select any random character but him.

Also sometimes automatic conversations happen, and usually with the character with the lowest Chr...

How I fix this? (specially the interface issue, that is more frequent)

When you start an interaction while the whole group is selected, I think there are two ways, depending on the action, the game decides upon the acting character: A) the nearest character and B) the last single character selected, before you selected the group.

For more control, you could select a single character before clicking on a subject (e.g. by pressing its shortcut number key) or you use the radial menu to choose an action. With the whole group selected, this results in the party leader doing it. The party leader is the portrait to the most left.

To avoid surprises in automatically starting conversations, you can keep an eye on the charismatic character being the last selected single character, before selecting the group.

Furthermore there were problems when choosing dialog options by pressing the according number keys. The game thereby also changed the selected character, but I'm not sure if this has been fixed meanwhile.

Ah, that explains it, I DO use the numbers when talking to people, and since frequently the quit dialog option is 3 or 4, I end selecting those.

Official solution: None, the game is buggy and random conversations with unwanted characters and you selecting people you don't want when using numbers on the dialog system will always happen.

EDIT: Oh crap, I cannot mark this as a solution. Well, this does not have a solution anyway.