Answer
Charade, a 1963 romantic thriller, featured Walter Matthau in a supporting role. The film follows the story of Regina Lampert (Audrey Hepburn), a young widow who becomes entangled in a dangerous game of cat and mouse with a group of men seeking her late husband's hidden fortune. Matthau played the role of Tex Panthollow, a bumbling private detective hired by Regina to investigate the disappearance of her husband.