It's the 50th Anniversary of Doctor Who this year, it originally screened its first episode in 1963, and there are many celebrations planned. Two of particular note are a dramatic recreation of the origins of the show, called An Adventure in Space and Time, starring David Bradley in the role of William Hartnell, the First Doctor; and a special episode of the regular series with the Tenth Doctor, portrayed by David Tennant, returning to dash around with the Eleventh Doctor Matt Smith.
A few years back there was an episode specially created for Comic Relief, where the Fifth Doctor encounters the Tenth Doctor. It was called Time Crash, and also served as a mini-prequel to the Christmas episode, Voyage of the Damned. Peter Davison still had enough youth in him to get away with it, though they were pushing it. But it made me wonder what you would do if you wanted any of the original seven incarnations to return to the modern series.
You can't use the original actors anymore, some are deceased, and the rest are just too old for it to be convincing. The earliest Doctor you can get away with is possibly Sylvester McCoy as the Seventh, but I'd limit it to Paul McGann as the Eighth. That means if you did want to bring any of the earlier Doctors back you'd have to recast.
And that made me wonder just who could portray each of them convincingly. They'd have to resemble them physically, be approximately the same age as the original actors were during their run, and also have a touch of their personality, or at least be good enough actors to do a decent impression. Their voices may also have to be dubbed by impressionists (or by their original actors), though perhaps that's something that doesn't really matter in the end.
David Bradley seems to be ideally suited as William Hartnell. I look forward to seeing his performance in the Special. I am not too familiar with the First Doctor, though, so really you could play fast and loose and I wouldn't notice.
Jason Isaacs may seem an unusual choice for the Second Doctor, but his resemblance to a younger Patrick Troughton is undeniable, so I think if he pretended to be shorter and had a bowl-cut wig, he'd do a fantastic job.
Jon Pertwee is a hard actor to find a double for. The best I could find was Geoffrey Rush, and it's a bit tenuous, but I figure if he had the shock of white hair and frilly sleeved velvet jacket the resemblance would be enough. He has certainly established himself as a great mimic, so can portray Pertwee's mannerisms with aplomb.
Update: My friend Jet Simian suggested versatile Australian actor, and contemporary of Mr Rush, Richard Roxburgh as the Third Doctor, and, by the powers, he's right!
Now this is a very strange choice. Our memory of the Fourth Doctor is of a tall hulking man, with a rictus grin and crazy eyes, a huge fright wig of curly hair, a brimmed hat, and a long scarf. But Tom Baker was not as fat a man as he later became. Frank Woodley is quite a thin lanky man, but is equally tall, has a manic look, and something of a resemblance to Tom Baker. If he put on weight, or at least a muscle suit, the other accoutrements would be enough to paint a convincing similarity.
A youthful blonde cricketer is just about all you need to get a decent resemblance to the Fifth Doctor. All Dan Stevens needs is the floppy hair and I think it'd be a successful transformation.
I struggled with the Sixth Doctor. Colin Baker has a very square head, dominated by his curly blond locks and his garish costume, and there aren't too many actors around that have that face shape. Mark Gatiss, creator of the aforementioned 50th Anniversary Special, and also co-creator and actor in Sherlock, and a regular writer for the current series of Doctor Who itself, is I think a close enough match to work just fine.
Sylvester McCoy got his start as a Music Hall performer, a comedian first and an actor second, and it's still deeply a part of his real world character even now, so what you need is someone with a similar sense of self but with a bird-like face and mannerisms. Milton Jones has a reasonable physical resemblance, but more importantly has a similar character in his comedy performances which makes him a good choice.
In the only instance where recasting occurred (in the story The Five Doctors where Richard Hurndall played the First Doctor), they got away with that because most viewers had no familiarity with the First Doctor, he was from before their time. Despite ubiquitous access to early episodes on DVD, the current audience, especially non-UK and the under 20s, are also not very familiar with the early incarnations of the Doctor. I think that means maybe, just maybe, recasting might work after all.