Director George Cukor suggested Inrid Bergman study the patients at a mental hospital to learn about nervous breakdowns. She did, focusing on one woman in particular, whose habits and physical quirks became part of the character. Bergman usually succeeded in getting her way during film productions, but she did lose on one important detail. The actress hated to begin shooting with a passionate love scene before she got to know her leading man better. But the first scene captured on film had her leaping out of a railway carriage and racing into Boyer's arms. It was an awkward moment for her, all the more so because Boyer was a few inches shorter than her and had to stand on a box for the scene. "I had to rush up and be careful not to kick the box, and go into my act," Bergman said in her biography “My Story,” written with Alan Burgess. "It was easier for us to die of laughter than look like lovers."

Bergman had great respect for Charles Boyer, however. In her autobiography, she called him the most intelligent actor she ever worked with and one of the nicest. "He was widely read and well educated, and so different," she wrote. Boyer's height presented more than one problem during the shooting. He had to perch on a box again when he next appeared with Bergman in Arch of Triumph (1948). And at 5 '8," Angela Lansbury, his co-star in Gaslight, was as tall as Bergman. Cukor made her wear platform shoes to increase her height and accentuate her sinister persona in scenes where she had to threaten Bergman. That only made Boyer's shortness (and need for a box) more evident.

One of the happy results of Gaslight was that it launched Lansbury's long and acclaimed acting career. In Peter Bogdanovich's book of interviews, “Who the Devil Made It,” Cukor recalled the casting of Lansbury: "...there was a very good part of a rather sluttish housemaid. We looked around, we saw some English girls, and they weren't really very fresh or quite right for it. And playwright John Van Druten, who wrote the script, said: 'You know, Moyna MacGill'—who was a very well known English actress—'is here with her three children. She was a refugee during the war and I know she has a daughter—I think she may be fourteen; I have no idea how old she is.' Then he found out and said, 'Yes, she is sixteen or seventeen.' At the moment they were working at Bullocks making Christmas packages and this girl came up who had never acted before, and she read the thing and I thought she was awfully good....Anyway, she did get the job. Now: the very first day on the set, she was absolutely at home—she had never acted. She wasn't as accomplished as she is now but she was an actress and she had the talent for changing herself physically without appearing to. And she had this rather sullen, bad-tempered face—rather impertinent face—it just came from the inside. And there was this full-blown character. Then what makes her interesting is that right after that she played in The Picture of Dorian Gray (1945)—directed by a friend of mine, Albert Lewin—in which she played the most exquisite and fragile heroine. That could have been awfully saccharine and she did it with great delicacy and feeling—and looked quite different." Lansbury made such an impression in her debut she was nominated for a Best Supporting Actress Academy Award and cast in another film that same year as Elizabeth Taylor's older sister in National Velvet. Many years later when she was watching the film again, Lansbury commented with astonishment, "My God, how did I have all that assurance?"

George Cukor was known in the industry as a "woman's director," and many of his best-known movies up to that point had been stylish and breezy comedies: Dinner at Eight (1933), Holiday (1938), The Women (1939), The Philadelphia Story (1940). But he also knew his way around period dramas: Little Women (1933), David Copperfield (1935), Camille (1937). Still, many were surprised and impressed by the skill with which he created the tense and moody atmosphere of Gaslight. "I really think the style comes out of the story," he told film historian Gavin Lambert late in his career. "If you're going to do a story about a murder in a Victorian house, you make it claustrophobic, you make it clouded and gas lit. You research the period, not just to reproduce things physically but for the emotions it stirs up in you. ... I always say the text dictates the whole style to me, which may not be to the director's advantage, because it means his touch is not immediately recognizable."

Cukor said he didn't like to talk about the part with actors too much because "you lose the magic" and that he never rehearsed the emotions of a scene, only the mechanics, so the actors could make fresh choices when the cameras rolled. But he also admitted that he had a tendency to "lead" the actors, in his own special way. While directing Bergman, he kept retelling her the plot to bring her up to the emotional point of the scene and keep her intensity up between takes. Finally one day, she told him politely, "I'm not a dumb Swede, you've told me that before." Cukor stopped telling her anything, the result of which, he said, was that the producer watching later rushes told him the actors appeared to be "acting as though they're under water." So, Cukor resumed his storytelling method, a practice Bergman soon grew to appreciate.

Years after the release of Gaslight, Cukor pointed out that his direction and the performances of the cast weren't the only factors that made the film a success. He also credited the rich production resources of MGM for access to all the items needed to create the film's Victorian home in minute detail, an important factor in a story that involved objects from the house being deliberately misplaced and "stolen" to convince Bergman's character she was going mad. Cukor told Lambert about Paul Huldschinsky, a German refugee whose family had owned newspapers and whose wife had once owned railroads in their native country. At the time of the film's production, Huldschinsky was working in a rather obscure job as a set dresser at Metro, primarily doing gas stations and other rather pedestrian assignments. The studio wanted to put one of their more established and well-known dressers on the project, but Cukor insisted on Huldschinsky and was rewarded for his support by an intricate and lushly detailed set that earned an Oscar.