Michael Redgrave's role here as Gilbert is in a similar vein to Robert Donat's in the 39 steps as they both are the male leads in Hitchcock thrillers. There are some differences though. The biggest being that Redgrave's role is technically secondary lead in that he does not get to be the one in the precarious situation. Instead Gilbert is the one who needs some convincing, and Redgrave never leads the film solely as Robert Donat had in the earlier film. Redgrave though definitely takes a similar approach to the material as Donat had though.
Before the thriller starts this movie is more of a lightweight comedy. Redgrave appears briefly in these scenes as another vacationer who likes to play his clarinet loudly in the hotel the vacationers are staying at, and after Iris bribes the hotel manager to kick him out he does not mind imposing on Irish until she has him reinstated. Redgrave's scenes are brief here and in these scenes he is almost entirely comedic in his style. Redgrave does this well, and plays Gilbert in a witty fashion that allows Gilbert's rather flamboyant actions play out in a fashion that never makes them too over the top.
When the thriller starts Iris chances upon Gilbert on the train they are taking back to England, and he decides to help her look for the missing old woman. Redgrave keeps a light touch in these scenes as well, but he is rather enjoyable in the tone he sets up for Gilbert. On one end he does add some nice bits of humor to the proceedings whenever he has a chance, and has some nice timing to add something humorous at just the right moment. At the same time though he never goes too far though to mess with the more serious moments, although Redgrave does appropriately leave Lockwood to bring the dramatic weight as Gilbert for a long time is not entirely convinced.
Redgrave does well to ease into a greater seriousness as the situation slowly becomes much more dire as they find themselves in the plot. Redgrave always keeps that lighter touch though even in the climax, but he does find the right tone and allows their predicament to be enjoyable as well as thrilling. Redgrave also does work well with Margaret Lockwood, and Redgrave handles the romantic angle particularly well. There is not too much focus given to it throughout the film. Redgrave is good in the brief moments where Gilbert suggests interest which are very much underplayed, but effectively so. When they do end up with each other it works out just fine.
Michael Redgrave gives a nicely handled performance here that serves this fun thriller well. I would say as Alfred Hitchcock leads of this kind I did prefer Robert Donat's work in the 39 Steps. That is not to say anything negative about Redgrave's performance though as I really liked Donat's performance in that earlier film I also quite liked Redgrave's performance as well. Honestly I found Redgrave's performance quite delightful the whole way through the adventure always bringing that nice comic touch, while being entirely convincing when it is necessary to adjust his performance for the more harrowing sequences.