@Nousher Ahmed,
They are both correct grammatically but they mean different things. The plural verb refers to both things, (the horror and the suspense) but the singular verb only refers to the previous single thing mentioned (the suspense).
1) The film is full of horror and suspense THAT KEEP the viewers spellbound.
1) The film is full of horror and suspense. They both keep the viewers spellbound.
2) The film is full of horror and suspense THAT KEEPS the viewers spellbound..
2) The film is full of horror and suspense. The suspense keeps the viewers spellbound.(The implication is that the horror does not.)
Consider:
The cake is full of nuts and fruit which are delicious.
The cake is full of nuts and fruit which is delicious.
Mary has skin and hair which are beautiful.
Mary has skin and hair which is beautiful.