It's because, at least for me, Marlene never goes into that stratosphere of magic except when she sings--and then she does. Garbo doesn't inhabit the realm of singer at all, and never intended to, but that's why Dietrich can 'stand up to her' as a persona (though not really as an actress ever IMO). My Garbo-fan friend mentioned her singing in something, but I don't know what that was, nd he can't seem to remember, in any case she wasn't a singer. You're absolutely right that Dietrich is made for cabaret, and she was the master of it. And that is the one time where she is totally unlike anyone else, even with the well-known limited vocal range that you mention. It is marvelous the way she can make you forget everything else going on in a film like 'Foreign Affair' when she sings 'Black Market'. Her praise of the years of working with Burt Bacharach in the 50s are truly moving. I happen to adore 'Blue Angel, I think it's her best film, actually. She is good as an actress in 'Witness for the Prosecution' and also very fine in that exotic role in Welles's 'Touch of Evil', a masterpiece of a film. I also love 'Blonde Venus', no matter how silly some of the plot line, because her numbers are so full of life and humour, she's just one of those naturally funny people (which might be a liability as an actress, because she often comes across as camp.) I don't like, however, 'The Devil is a Woman' and 'The Scarlet Empress' is just ridiculous. Also like the two Cooper/Dietrich films you mentioned, as well as 'Shanghai Express' and when she sings in 'Destry Rides Again'. Mae West had another version of this, although hers was the surprise (as in 'Belle of New Orleans') when she starts singing, and it's so effortless, and you just weren't expecting it to be so good. You probably know the hilarious stories of Dietrich and Mae, as well as the one of Mae and Garbo, in which I believe Garbo stood silent almost the whole time while Mae talked about her career non-stop.
I never considered her to be Garbo's "riva"l, although one can understand why people kept comparing them and trying to put one against the other (which is absurd since there is place for both of them and they are totally different both as actresses and personalities).
This is marvelous:
'what memories used to be about...' ah yes, that is quite perfect, I'd like to have written that myself, do you mind if I plagiarize?
She was this striking personality who goes through the decades and despite the marks and passage of time she touchingly reminds audiences what memories used to be about.
I like this also very much:
That explains it just a wee bit more vis-a-vis the necessity of film for her gifts than I've heard it said before, makes it truly singular. Deneuve has only made films (I was surprised when I read this) and her favourite actress is Marilyn Monroe, and it is interesting to think of how some actors and actresses are made purely for film. And in recent years, I've noticed that the bigger film stars of today don't usually succeed so well onstage--Julia Roberts, Julianne Moore, there are others I can't think of right now; Keith Carradine was marvelous in 'Will Rogers Follies' on B'way, but never quite made it to 'Major Star', part of which can be explained by his poorly executed British accent in 'The Bachelor' with Miranda Richardson; otherwise, I never could figure out why the momentum didn't keep going, because that's a real talent (he's even good in that Madonna 'Material Girl' music video--and one of his very best roles is relatively late: His big fist fight with Vanessa in 'Ballad of the Sad Cafe', a film usually disliked, but which I found very impressive, and Carradine is sensational in it.) Only Vanessa, who is primarily known as a film actress, is actually even greater when you finally do see her onstage--if I had to choose between Deneuve's film work and Vanessa's (as whole bodies of work, esp. including recent years), I'd definitely take Deneuve's, who has gotten better as she's aged. After what you said about Seyrig (and that I should have known), I wonder if her stage appearances were as great as her film work. But the perfect 'diagram' of her film luminosity is in 'La Peau d'Ane', in her scenes with Deneuve: In no catty way, she totally dominates the scenes as the Lilac Fairy; and it comes as little surprise that Deneuve, as charming as she is, doesn't resent this at all--it's clear she understands that Delphine was capable of a luminosity that she didn't even have to work for, and she's just dazzling as the Lilac Fairy in that charming film.
[Garbo] was this artist of genious whose's seduction and genious would touch the audience only when she performed a part and through the camera lens.
Barrault also had that luminosity, even if we know him only from 'Les Enfants du Paradis', even though he was primarily great for his stage acting. But Marie Casares, from the same film, we'd hardly know at all from it, and then we hear of her illustrious career at the Comedie Francaise (I believe it was that.)