Getting acting jobs for feature films may be your dream, but doing commercials could help you break into the industry. There is some real money out there for those who can build up a good resume of this type of work. Every time that commercial airs, you get a residual payment out of it.
Sounds great, doesn't it? Getting paid even after you're finished with the job.
If you do decide to pursue commercial acting, you need to master the differences between the two flavors. In commercials, you almost always use cue cards to read off of because the dialogue can constantly change during the audition. For this reason, memorizing the lines is not a good idea.
When you go into an audition, don't wear a costume of any kind, even if the role seemingly calls for one. It will distract from your performance and make you look silly at the same time. Of course, if the role calls for a housewife, you don't want to go into the audition wearing a "power suit."
Auditioning for a commercial requires one to listen to the casting director. If the actor is so nervous or excited that they're not paying attention, they will miss any verbal cues that what they're doing isn't working. Relax and pay attention.
Always remember that if you decide to get a commercial agent, don't neglect your work towards getting a theatrical one too. The contacts you meet on a commercial could help you in getting a film role.