My PC's were warned that there was some nasty tentacled creature that was very tough somewhere in the lower levels. So when they walked by and it grabbed two of the party, they knew they couldn't beat it. I made it talk as soon as it grabbed the two party members ("Yum! Fresh food!!"). This told them that conversation (and therefore bargaining) was possible as well as giving away it's primary motive for grabbing them.
As it held the two adventureres underwater in the river, the party went into frantic diplomacy mode. They promised to trade it lotsa food (the bear from the trog pen, orcs, anything!) if it would not kill or eat their friends. It thought for a second, then pulled the two gasping out of the water and slammed them into the wall and into unconsciousness. With the clock ticking, the rest of the party went to find the food they had promised. Soon enough they returned and the roper threw their two friends across to them and they ran away.