Face the NPC. Stand ****ing still. Press ctrl+forward. PROFIT.
Two buttons to talk to people? OH LORDLY LORD.
Well, in Gothic 1 you do this while interacting with *everything* in the game world. Sooner or later you will get used to it. Gothic 2 made it a bit better tho'. I personally never played G1 or G2 with mouse and keyboard- always only with keyboard, because that's how I knew it from G1. Everything else felt wrong to me.
I was one of the few who wanted the original controls even in Gothic 3, but only because the mouse controls in Gothic 2 felt ****ty to me and yeah, if this would become the default way...
However, the keyboard + mouse controls in Risen are pretty good, imo. Don't want it to get changed anymore now.