How to present yourself for a commerical casting
All types of castings are different. A casting for theatre will be nothing like a film casting or a commercial casting. Castings are quite strange situations to be in anyway, and commercial castings are probably the most bizarre.
The main thing with any casting is to be yourself. Unless you're really miserable then don't be yourself - you could write a whole different article on this - but the general consensus is to come across as happy, friendly and willing.
Castings for commercials are often compared to being in a cattle market - in the initial audition stages there are usually a lot of actors there. Sometimes they all look similar - maybe blue eyes and blonde hair - the casting director/producer/client knows what they want for their product. However, it's just as likely to find yourself in a room with people of different heights, gender, race, hair colour, eye colour, piercings, tattoos and any other difference you can think of.
Commercials aren't long so the characters are often stereotypes in order to instantly engage with the audience. If the role is for a builder, it's likely that they've asked a lot of bald, harder looking men to audition. So how do you physically present yourself? You don't need to go out and buy a hi-vis jacket and a hard hat - that's probably a bit too far in my opinion. But there are subtle things you can use to emphasise that side of you. For example, you may have a leather jacket or a pair of boots that you could wear to make you appear a bit rough around the edges and more 'builder/bouncer like', possibly making it easier for the casting director to picture you as a builder.
You may have noticed commercials tend to be caricatures of real life and somewhat unrealistic - from cats attacking people to spoons having faces and talking. That requires a slightly different mindset for the whole process. This means you should definitely present yourself as open to anything - not anything and everything - but open to making stupid faces or making a fool of yourself a bit because you've been asked to talk to an imaginary gremlin running around your feet. You may feel stupid at the time, but everyone else in the waiting room is about to do it as well. If you commit to whatever it is they ask, then it doesn't look stupid and the audience will go with you. However, if you look up and smirk at the director every 10 seconds because you feel like a fool, then you'll look like a fool. A good logical point to remember is: don't worry about looking silly, you probably wont ever see these people again. If you do see them again then it's because you got the job and they liked what you did.
After you've presented yourself in a friendly way and you've nailed the audition with confidence and were not worried about looking like a fool, the situation is out of your hands. You've done your best and there's nothing more you can do. You might see the commercial being broadcast in a few months and think you did a better job than whoever it was that they cast - and you probably did. Although your acting was just what they wanted, you might have had a slightly different look to what they wanted. Commercials are very subjective and ultimately the big brand client is paying for it. If they don't want someone associated with the product just because their hair is slightly too dark or their nose that little bit too big, then they wont sign off on the production. There's nothing you can do.
Don't forget, the person who takes your name or directs you up the stairs or gets you a drink is probably part of the team - if you're rude to them before you even audition then you might as well have just slapped the director in the face.