In all the origin stories and postmortems for the various eras of Saturday Night Live, the show’s decision to employ guest hosts has long been a bone of contention. Albert Brooks claims that he off-handedly suggested to producer Lorne Michaels that SNL bring in a new host every week, after Brooks rejected an offer to be the show’s permanent frontman. (Michaels disputes this account.) Later, in SNL’s 10th season, when Michaels’ successor Dick Ebersol decided to revamp the series by hiring established writer/performers like Billy Crystal, Martin Short, Christopher Guest, and Harry Shearer, there was some public grousing from the new cast—Shearer especially—about the ridiculousness of pairing skilled sketch comedy pros with the likes of Jesse Jackson and Bob Uecker.


At this point, as Saturday Night Live gets set for its 40th anniversary special this weekend, there’s zero chance that Michaels—back in charge since 1985—is going to ditch the entire concept of the guest host. SNL is wedded to a formula, and a big part of what makes the show culturally relevant is how it gives popular actors, comedians, musicians, athletes, journalists, and politicians alike a chance to prove that they can make (and take) a joke.


It’s not always easy to predict who will soar on Saturday Night Live though, and who will stumble. Who would’ve guessed that Justin Timberlake would practically become an SNL regular? Or that a talented actor and funny guy like Bryan Cranston would come across as awkward and flat the one time he hosted?


Making an impact on the show requires a particular set of skills. Over the past 40 years—and especially over the last 10 or 15, as Saturday Night Live has become increasingly slick and machine-tooled—the success of a host is largely attributable to three factors, in roughly this order:

1. The ability to read

Saturday Night Live has traditionally been unapologetically cue-card-reliant, to the extent that during Harry Shearer’s first stint on the show in season five, he reportedly irritated his fellow cast members by showing up prepared even to rehearsals—as though he were implicitly calling them lazy. But while Michaels later loosened up, in the early years he established a value system for SNL that frowned on ad-libs and breaking character; and whenever the hosts and cast aren’t focused intently on reading their lines, the odds of a slip-up increase. The upside to the Reading Is Fundamental ethos is that while SNL has good weeks and bad weeks, regular viewers have come to expect a baseline of competence. Saturday Night Live is rarely painful to watch, in other words. The downside is that the sense of daring and deep generational connection that SNL carried through its first few seasons is long gone. The people onstage often seem more engaged with the cue cards than with each other or with the audience.


Some hosts are better readers than others. Kevin Hart is a funny comedian, but when he hosted SNL recently, he raced through his lines so quickly that it was hard to understand some of the jokes. Blake Shelton did okay as host a couple of weeks ago, but read his lines in a kind of twangy monotone that made every one of the characters he played come across as more or less the same. The people who’ve done surprisingly well on Saturday Night Live in recent years—like Drake, or Anne Hathaway—brought a real vitality to their line-readings, coupled with a willingness to adjust their voices to the characters they’re playing. Which ties into the next key hosting quality….


2. Gameness

Whenever Saturday Night Live celebrates itself, the clip packages generally emphasize the cast’s recurring characters, but one of the few one-off, host-driven moments that pops up again and again is the William Shatner sketch where he addresses a Star Trek convention and tells Trekkers to “get a life.” The quickest way for SNL hosts to rally the audience to their side is to make fun of themselves and their work, whether it’s Lady Gaga winking at her own pretensions or Martin Freeman appearing in a sketch that riffs on both The Office and The Hobbit.

Being “game” as an SNL host isn’t limited to having a self-deprecating sense of humor. From Rudy Giuliani wearing a dress to Howard Cosell sporting an Ed Grimley outfit (and hair!), the willingness of hosts to risk looking like an idiot is essential. The worst Saturday Night Live hosts aren’t the ones who can’t emote—a lot of non-actors who suck at reading with inflection still do fine on SNL—but the ones who end up playing the straight man in every sketch, because they’re either unwilling or unable to be wacky.


3. Talent

Again, it’s not essential that hosts be able to act, and it’s not all that important that they be naturally funny. The best actors are sometimes lousy on Saturday Night Live, perhaps because they’re overqualified, and have difficulty to adjusting to scene partners who are constantly staring over their shoulders at cue cards. Stand-up comedians too have been hit-and-miss as SNL hosts. They deliver outstanding monologues, but they look a little lost in the sketches, where they have to let other people get laughs.


Still, while personal charisma is in some ways the most valuable asset an SNL host can have—it’s why athletes have so often thrived on the show—the best hosts are the old-fashioned triple-threats, who can sing, dance, and deliver a joke. Two-time host Joseph Gordon-Levitt is a good case-in-point: a likable guy who makes an honest effort to entertain every time the camera points his way. Ditto Anna Kendrick, Anne Hathaway, and Justin Timberlake. These are people who are comfortable performing, and bring a refreshing gusto to everything they do. They could’ve been stars in any era.


There’s a danger though in becoming too comfortable on Saturday Night Live. Good hosts keep getting asked back, and have proved they’re no fluke. Tom Hanks, Christopher Walken, John Goodman, Alec Baldwin… They’re all in that SNL hosting sweet spot, able to come across as relaxed and egoless. But there’s an element of diminishing returns even to the best of the repeaters, similar to how even the funniest Saturday Night Live characters run their course after a while. At this point, if Timberlake were to host SNL again, fans of the show would have a pretty good idea of what to expect, right down to his pushy corporate mascot shtick and the inevitable return of Jimmy Fallon for another Barry Gibb Talk Show. Even Melissa McCarthy, who’s frequently been hilarious in her three SNL hosting stints, started to hint that she was repeating herself by round three.

This has long been the grand paradox of Saturday Night Live: The show’s popularity has been built on repetition, but its original reputation as a home for cutting-edge comedy takes a hit every time the writers and cast trot out another installment of What’s Up With That? There’s something comforting about the familiar SNL structure—cold open, credits, monologue, fake ad, sketches, music, Weekend Update, sketches, music, super-weird sketches, goodnight—but also something dully conservative about it.


New cast members can do a lot to change the energy. (Lately, Leslie Jones has lit up the screen nearly every time she’s appeared.) But the biggest difference from week to week really has to be the host. It’s all well and good for a Blake Shelton to do a respectable job and then to celebrate over the closing credits with an awestruck, “I did Saturday Night Live!” But dating back to the show’s first season, when Richard Pryor hosted and conferred a new level of credibility to SNL, some of the most memorable moments have come when a host has been so strong that they have altered the entire dynamic of the room. The “just fine”-ness of much of Saturday Night Live may be holding too many potentially great hosts back. The show is too foolproof these days to allow guests to fail miserably. But is it still open enough to let one drop by and dominate?