 Alec GuinnessProfessor Marcus | Professor Marcus |
 Cecil ParkerClaude, alias 'Major Courtney' | Claude, alias 'Major Courtney' |
 Herbert LomLouis, alias 'Mr. Harvey' | Louis, alias 'Mr. Harvey' |
 Peter SellersHarry, alias 'Mr. Robinson' | Harry, alias 'Mr. Robinson' |
 Danny Green'One-Round', alias 'Mr. Lawson' | 'One-Round', alias 'Mr. Lawson' |
 Katie JohnsonMrs. Wilberforce | Mrs. Wilberforce |
 Jack WarnerThe Superintendent | The Superintendent |
| PPhilip StaintonThe Sergeant | The Sergeant |
 Frankie HowerdThe Barrow Boy | The Barrow Boy |
| MMadge BrindleyLarge Lady (uncredited) | Large Lady (uncredited) |
| HHélène BurlsHypatia (uncredited) | Hypatia (uncredited) |
 Kenneth ConnorCab Driver (uncredited) | Cab Driver (uncredited) |
| MMichael CorcoranBurglar (uncredited) | Burglar (uncredited) |
 Harold GoodwinParcels Clerk (uncredited) | Parcels Clerk (uncredited) |
 Fred GriffithsJunk Man (uncredited) | Junk Man (uncredited) |
 Lucy GriffithsMiss Pringle (uncredited) | Miss Pringle (uncredited) |
| GGeorge HilsdonSecurity Van Driver (uncredited) | Security Van Driver (uncredited) |
| VVincent HolmanStation Master (uncredited) | Station Master (uncredited) |
 Stratford JohnsSecurity Guard (uncredited) | Security Guard (uncredited) |
 Sam KyddSecond Cab Driver (uncredited) | Second Cab Driver (uncredited) |
 Edie MartinLettice (uncredited) | Lettice (uncredited) |
 Jack MelfordDetective (uncredited) | Detective (uncredited) |
 Arthur Mullard(uncredited) | (uncredited) |
 Ewan RobertsConstable (uncredited) | Constable (uncredited) |
| GGeorge RoderickRadio Car Policeman (uncredited) | Radio Car Policeman (uncredited) |
| JJohn RudlingNervous Man (uncredited) | Nervous Man (uncredited) |
| LLeonard SharpPavement Artist (uncredited) | Pavement Artist (uncredited) |
 Peter WilliamsDetective at Parcels Office (uncredited) | Detective at Parcels Office (uncredited) |
 Neil WilsonPoliceman (uncredited) | Policeman (uncredited) |
| PPhoebe HodgsonConstance (uncredited) | Constance (uncredited) |
