The guy shouldnt see it coming if he is in danger of something else.
A basic set up.
1.Get good control of his arms. (double wrist control. double elbow control. wrist/lapel)
2. Open your guard, pull him into you and place your feet on his hips (dont forget to squeeze the knees)
3. If you got good control of his arms this right here should break him down a bit.
4. If it doesnt break him down, or he posts on a leg, than kick out the knee that is on the ground.
5. This will have him off balance and you can go for a sweep, or go for a regular triangle.
Check out the grappling FAQ for a basic triangle explanation that all NOOBS should read even though none of them seem to.
All my triangles are offensive. I dont do them as a reaction to a guardpass anymore.
I feign a sweep to break them down, or attack an arm and make them fight to pull it close. Than I go for my true goal. The triangle choke.