A lot of TV brackets have quite wide mounting plates, so you can often get at least one decent size wood screw into a stud. Then use suitable plasterboard fixings for the rest.
I've used the 'Grip It' fixings a few times and they seem pretty good. Note, I've no link with this company. I always fancied trying them since I saw them on Dragon's Den though. They were one of the better ideas on that program!
Obviously if it's a huge telly, or an extending arm then you will probably need to do what other people have said, ie. insert some plywood or a noggin.