The Horse's Mouth is an acting and a writing tour de force for Alec Guinness, who authored the screenplay in addition to starring in the film. Gulley Jimson (Alec Guinness) is an aging artist with a reputation as a genius, though he lives an impoverished life. Jimson has reached the point in his life where he no longer feels any need to moderate his irascible persona -- he has a taste for alcohol and a tendency toward boisterous spirits where the ladies are concerned -- in search of canvasses to paint and commissions that will allow him to live comfortably, and Guinness lives the role to the hilt. Released from jail for some indiscretion, he immediately begins harassing his wealthiest patron, Hickson (Ernest Thesiger), for money. When that fails, he insinuates himself into the home of a would-be patron, Sir William and Lady Beeder (Robert Coote, Veronica Turleigh), and manages to destroy their home and that of their downstairs neighbor with a huge block of stone and some help from a sculptor friend (Michael Gough). Courted by a potential buyer, he is desperate to retrieve one of his early works from his former wife, but even that prospect is closed off to him. Finally, with help from his young admirer, Nosey (Mike Morgan), his friend, Coker (Kay Walsh), and some art students eager to work with the legendary Gulley Jimson, he begins painting his largest canvas of all. The painting is completed and promptly destroyed. Jimson finally takes off in his wreck of a houseboat for the open sea, eyeing the huge hulls of the passing ships as potential canvasses to paint. As he disappears up the river, Coker looks on in panic and Nosey calls after him, declaring his admiration for Jimson and who he is and what his work means -- knowing for certain that he can't be heard.
~ Bruce Eder, All Movie Guide