Whoopi was inspired casting. She's perfect for the role; look and listen to her body language and demeanor. She's already an award winning actress, but what she brought to the role only expands her range in acting - and elevated Guinan as a result. A known actor or an unknown can both bring something compelling to a character, but could I imagine anyone else as Guinan? Not really. Whoopi definitely made Guinan her own in the same way Christopher Reeve made Superman his, Lynda Carter for Wonder Woman, Tobey maguire as Spider-Man, and so on. They're definitive. And not all were big stars at the time, so it is more about the talent of the actors and what they see in the character to draw out. IMHO of course. Can a movie have too much star power? Look at the Star Wars prequels. Lots of big names for sure but, for the attempts to expand the lore, the movies largely fall flat. So there's far more to media than just an actor's name or the idea that only big actors can instantly make any character great, like the writing and direction for the actors to take their ideas forward with (or be hindered by). But I digress.
Topped with some top-notch writing on top of Whoopi seeing much in the character (and she did want to be in the show, for which a lot of us are highly appreciative of), Guinan quickly became a memorable character, even eclipsing Troi in a role that doubles as a counselor-type figure. Along with Data, she's one of the most interesting Enterprise-D crew. And gets some of the coolest scenes and episodes. We get to see hints of a backstory with Q, with Picard (especially in TBOBW, how he's more than mere family - which is very open-ended, allowing the audience to think of many possibilities)... She's definitely got a 6th sense with dimensions and causality - right down to Yar and Sela (a terrific sci-fi creation that gets underused and misused and not living up to the potential thanks to poor writing, Denise Crosby did wonders with what she'd been given.)
That and, prune juice.
Loved how she got Worf to try it. No questions, just get him to open his mind.
She'd also dealt with the Borg (though 'I Borg' shows her character being altered to fit the story narrative, which doesn't fit compared to what has been seen prior to. It's possible she'd change her mind but her epiphany seems too quick. I should re-watch the story, I might not have picked up on something.
Her outfits were as much "accessible" as they were "alien" and Whoopi carries them off with aplomb. I love the style, especially the hat. The Enterprise was the allegory of a melting pot, so there had to be differences as opposed to a universal uniform standard, and Guinan herself adores half-bucking the system, especially with her reaction to Ro's introduction (one of my all-time favorite scenes ever in TNG), so either way the outfit is germane. (DS9 would also get to do a lot more with this, on both counts.)
Around season 6, I was asking where Guinan was since she'd largely disappeared from the show and so much about her, Q, Picard, and other issues were left unanswered.
I wish she got better material in "Generations", since the explanation for her species is poorly handled, little more than empty fanservice, and poorly handled empty fanservice.
And her cameo in "Nemesis" was pretty much throwaway as well.
Not only was Guinan poorly handled, if the Borg destroyed the El Aurian civilization, how long were they traveling toward (Federation space circa the 23rd century, 78 years earlier and all.) Were the Borg already checking out the Federation? Or were they refugees for decades?
