Click on someone with the crosshair to use the item on them. In this case, they will eat the apple. There's no button to open backpacks directly, but 'Z' will open status windows for each character. Pick things up by dragging and dropping onto a character or inside their inventory. Use any items with double left click. Hold right click for movement - the closer the cursor is to the edge of the screen, the faster you'll go.

In Serpent Isle, 'B' opens the spellbook, 'F' will automatically find food in your inventory to fully feed someone if possible, 'M' opens the map, 'K' uses the keyring if you have it, 'P' uses a lockpick, 'W' uses your pocketwatch, 'L' opens a group status window, and 'S' opens the save/load menu.

There's a few other keys that you probably won't bother to use. Check the reference cards.