One of American Dad!'s best elements is Sir Patrick Stewart's reoccurring role as Deputy Director Avery Bullock, Stan Smith's immediate superior at the CIA. He is an increasingly bizarre character anchored by a committed performance, who foretold the overall evolution of American Dad! as a whole.
When American Dad! began, the series was more of a straightforward animated sitcom. It was initially set up as a more down-to-earth series, with Stan Smith and his daughter Hayley's differing politics being a consistent throughline with less of the overt absurdities that came to define the show. Part of Stan's work with the CIA was Deputy Director Avery Bullock. Played by Patrick Stewart, Bullock was first portrayed as a serious figure, contrasting against Stan's antics and more or less serving as the teacher to a bunch of surprisingly rowdy CIA Agents. However, from the start, there were hints of the more selfish and silly character Bullock eventually evolved into.
Season 2's "Bullocks to Stan" saw the Deputy Director briefly begin a romance with Hayley, and later order Stan to assassinate Jeff when Hayley left Bullock to resume her relationship with him. Likewise, Season 3's "Four Little Words" teased just how ridiculous Bullock could be, with "Stan introducing his boss to a new romantic interest" storyline resulting in multiple attempts on Francine's life. As the series has progressed, Bullock further indulged in this wild side. After being returned to America with some fanfare in "Four Little Words," Season 7's "Ricky Spanish" revealed that Bullock had completely forgotten his wife -- even after she'd been killed at the title character's hand -- and was fully enjoying himself while doing drugs and firing guns with an unnamed woman in a bikini.
What ultimately makes Bullock work as well as he does is Stewart's performance. A veteran of the Royal Shakespeare Company who's been nominated for many prestigious awards, and the heart of popular genre fare like Star Trek: The Next Generation and the X-Men films, Stewart has an almost unrivaled wealth of dramatic experience as an actor. But American Dad! takes all the natural gravitas and charm he imbues into his performances and channels it into the most ridiculous ideas possible, creating an absurdly petty, silly and committed comedic character. Storylines like Bullock becoming a DJ in Season 11's "Standard Deviation" wouldn't work without Stewarts' committed performance of a middle-aged man completely throwing himself into the world of being an underground musician.
Stewart brings the same level of authenticity to Bullock as he does Jean-Luc Picard or even his Shakespearean roles, allowing American Dad! to fully embrace a weird style of comedy that benefits from his grounding presence. Stewart even once spoke about his experience working on the show during an official appearance at the Oxford Union, where he described the process and his appreciation for American Dad!'s ability to be effortlessly productive and hilarious. In many ways, Bullock's development into such a figure can be seen as a mutual evolution of the show as a whole, which, after the initial two seasons, incorporated more unexpected storytelling and a willingness to get weird in a way few mainstream shows will.
It even extends beyond Stewart's role as Bullock. In Season 8's "Blood Crieth Unto Heaven," Stewart is at the center of an episode based on a stage-play adaptation of American Dad!. While Stewart appears as Bullock within the highly meta episode, he also appears as himself -- or more accurately as a self-aware parody of himself. He pretends to fall asleep on camera to highlight his acting skills, calls himself a "theatre genius" and admits to eating several of the plays out of sheer jealousy. Stewart's performance, and the willingness the producers had to embrace this weird and wild character with such a committed thespian, indicate the show's growth from an average animated series into something far more unpredictable, hilarious and enduring.