Miley Stewart is played by Miley Cyrus. Averted at first, as the lead role was named Zoe Stewart, later changed to Chloe Stewart because Zoe was thought to be too similar to Zoey 101, and the character was established well before Cyrus came in. In fact, she originally auditioned (yes, auditioned) for the role of Lilly. It was changed to Miley as all of the aliases were becoming confusing to her in Real Life. Still an odd subversion because Miley wasn't her real first name — it's Destiny Hope Cyrus, and Miley was a nickname she got as a baby — but then she had her first name legally changed to Miley.
Played straight in the case of Heather Truscott, played by Heather Locklear.
Jason Earles, who plays Jackson, was a real-life example of Vague Age, his year of birth variably being reported as either 1977, 1985 or 1988, until it was finally confirmed to be 1977, making him almost 29 years old when the show premiered. Averted with the rest of the young characters, who are played by real teenagers.
Of special note: In the episode which teased a Lilly/Jackson pairing, Emily Osment was 16 and Jason Earles was 31, or almost twice her age.
Season 4's recurring guest character Siena, implied to be around Jackson's age, is played by 26-year old Tammin Sursok.
Actually her age was never mentioned. There's no reason she can't be 26.
Defictionalization: Miley Cyrus' early music career consists entirely of songs from the show, including performances as Hannah Montana in real life. Arguably, this wasn't the original intention. Disney Consumer Products failed to produce any official Hannah Montana merchandise for the first holiday season following the show's initial airing in March 2006, indicating that they did not anticipate the popularity it would garner nor the demand it would generate.
Executive Meddling: Possible — In some of the episodes, there's an underlying feeling that the plot and even the show could have been brilliantly satirical, but that the Disney sitcom formula, as well as the target demographic being little girls, puts a limit on the writers' storytelling potential.
In the concert film, if you look hard enough, you might just spot Caroline Forbes as one of the backup singers.
To a (somewhat) lesser degree, Miley's friend, dancer/singer/choreographer Mandy Jiroux, as of March 2013 an emerging dance-pop/EDM artist under the name of MJX, worked as a dancer for Miley/Hannah tours and appearances for a number of years. By 2009 she appeared as part of YouTube web series The Miley And Mandy Show for a year, and for a period was a member of Girl GroupBG 5 (later rebooted as YLA without Mandy) before striking out on her own.
The Other Darrin: The role of Mr. Dontzig has alternated between twin-brothers Paul and Peter Vogt.
Real-Life Relative: Miley Stewart's father Robbie Ray Stewart is played by Miley Cyrus' father, Billy Ray Cyrus. (Yes, that Billy Ray Cyrus).
Reality Subtext: The fourth season deals with themes such as whether Hannah's audience would accept the change and growth in her work, Miley and Lilly growing up and out of the tastes and fashions they had when they were younger, and whether her fans would stay with her even after Miley stops being Hannah Montana and appears in public as the real Miley. This seems to reflect Miley's image change into a more adult and more edgy/personalized/reflective image and music style in Real Life, which began before the fourth season was filmed.
The show began as a That's So Raven spinoff called Better Days, about a child actor masquerading as a schoolgirl, and it was to star Alyson Stoner.
The lead character was to be named Zoe Stewart, then Chloe Stewart as they felt "Zoe" sounded too much like the lead in Zoey101. They settled on "Miley Stewart" so 13-going-on-14-year-old Miley Cyrus wouldn't have to remember so many names.
At one point the series was under the working title "Alexis Texas". Whether or not the change had to due to a porn actress of the same name who also made her first appearance in 2006 likely remains a mystery (but given Disney Channel sensibilities, it's a safe bet).