Film opening: Saw

Saw

The opening scene of saw starts with the title first followed a man choking on water in a bath tub the water drains out and we hear chains as the man climbs out of the bath tub. the man clearly scared starts shouting for help a calm older voice tries to answer the questions "what is that smell?" suddenly lights turn on and we see to men chained in a bath tub looking at a dead body with a gun in one hand and a tape recorder in the other. the set is all old and disgusting and blood all on the floor around the body. 

This opening scene doesn't fully indicate any genre but horror does come through body on the floor disgusting set. questions about the characters. this opening creates alot of enigmas. 

who are the two guys? why is there a body on the floor? how did they get there? why is there a tape recorder?