It seems network pilots (ABC, NBC, Fox, CBS, CW) start auditioning actors around Jan-Mar. Starting with series regulars then casting recurring and supporting roles next. Filming of the pilot episode usually begins mid-March. Then in May, there is a big announcement during what is called a "pilot upfront" where the public finds out which shows will be picked up and start airing during fall/winter season. Pilots also cast throughout the year, however, but the bulk of the talk you are seeing and hearing about now is for network pilots hoping to be ordered to series in May.
My DD did a pilot once. It was for Fox network and here is how it worked:
-Agent got us an audition appt. I guess same way they get any other audition appt - submit and/or pitch her when they are made aware of the project. I think as long as you have a theatrical agent, you have a chance to get a pilot audition.
-Appt was the last day in February. It was with CD to go on tape. At CD's office. Typical audition. Maybe 20 girls who seemed age 5-8 going for role of 6y/o daughter of the leads.
-First week of March we were asked to come in for what they called a director's session. DD was to meet with CD, Director, and Warner Brothers TV Casting at this appt. The appt was at the production office this time and seemed more intense as there were lots of adult actors there who were called back for adult roles. Very few kids present at this director's session. 20 little girls narrowed down to a handful.
-Second week of March, DD was asked to come in a third time. This time they called it a network session/network test. Fox network had to approve is what I guess that meant? That appt was at the CD's office again. I believe the network asked the CD to work on a few things with DD and put it on tape for them. They had her doing a few lines different ways and
-Few days later, DD was what they called "pinned" for the role. Something like a hold. I don't hear that term often but that's the term they used.
-Few days later, in mid March she was offered the role. She began filming the next day. Yes, they kept us all in suspense until the very end! It was a recurring role. Not a series regular. Maybe series regulars go through much more or totally different process.
-In May, all of the reports started being released and during the official "pilot upfront" announcements we found out the pilot was not picked up. With American Idol and X Factor, there wasnt much room on Fox's lineup for new shows anyway so I knew the chances were slim but I tried to stay hopeful. Top notch director, producers, and Oscar winning actor as lead...but it was a no go.
Here is a small excerpt from an article that explains how the pilot audition process works... http://en.wikipedia.org/wiki/Television_pilot
"Casting is a lengthy and very competitive process. For the 1994 pilot of Friends, casting director Ellie Kanner reviewed more than 1,000 actors' head shots for each of the six main roles. She summoned 75 actors for each role to audition, then chose some to audition again for the show's creators. Of this group, the creators chose some to audition again for Warner Bros. Television executives, who chose the final group of a few actors to audition for NBC executives; as they decide whether to purchase a pilot, network executives generally have ultimate authority over casting. Since the networks work on the same shared schedule, directors, actors, and others must choose the best pilot to work for with the hopes that the network will choose it. If it is not chosen, they have wasted their time and money and may have missed out on better career opportunities.
Once they have been produced, the pilots are presented to studio and network executives, and in some cases to test audiences; at this point, each pilot receives various degrees of feedback and is gauged on their potential to advance from one pilot to a full-fledged series. Using this feedback, and factoring in the current status and future potential of their existing series, each network chooses about 4 to 8 pilots for series status. The new series are then presented at the networks' annual upfronts in May, where they are added to network schedules for the following season (either for a fall or "mid-season" winter debut) and at the upfront presentation the shows are shown to potential advertisers and the networks sell the majority of the advertising for their new pilots. The survival odds for these new series are low, as typically only one or two of them survive for more than one season"