What do I do if someone at the gym is working a circuit and I need to use a machine that person is using?

This morning at the gym I needed to get on the triceps machine. I saw there was a guy who was working a circuit of about 4 machines, one of which being the triceps machine I needed to use. After he left the machine, I went and used it. The guy saw me on it and gave me a kind of dirty look. I guess I don't know the etiquette of they gym -- should I have skipped that machine or waited until he looked like he was done with the circuit, or was it okay for me to use the machine?