- Well, I suppose you know what you're doing.
- Oh, yes.

5 seconds sound clip from the The 39 Steps (1937) classic radio play.

You can hear this line at 00:25:43 in the radio play.

Quote context

[...]

- Oh, don't worry, I don't hold it against you really.

- Well, now, that's very kind of you.

- Yes, isn't it?

- What are you doing in Kinreach?

- I live here.

- It's a charming place, isn't it? You know where the local police station is?

- Oh, of course I do.

- Will you take me there?

- The police station. In a hurry, please.

- Certainly.

- Well, I suppose you know what you're doing.

- Oh, yes.

- You see, I know who killed Annabella Smith.

- Is that so?

- The same man who shot me a few minutes ago.

- Shot you? Where?

- Right in the cigarette case. See? There's a hole in one side of it.

- Handy little things, cigarette cases. If I hadn't been carrying it about in my vest pocket, the hole would have been in my chest.

- And who was this arch-villain?

- Gentleman called 'The Professor.'

[...]