Dracula

Renfield (Dwight Frye) is a solicitor traveling to Count Dracula's (Bela Lugosi) castle in Transylvania on a business matter. The people in the local village fear that vampires inhabit the castle and warn Renfield not to go there. Renfield refuses to stay at the inn and asks his carriage driver to take him to the Borgo Pass. Renfield is driven to the castle by Dracula's coach, with Dracula disguised as the driver. En route, Renfield sticks his head out the window to ask the driver to slow down, but sees the driver has disappeared; a bat leads the horses.
Renfield enters the castle welcomed by the charming but eccentric Count, who unbeknownst to Renfield, is a vampire. They discuss Dracula's intention to lease Carfax Abbey in London, where he intends to travel the next day. Dracula hypnotizes Renfield into opening a window. Renfield faints as a bat appears and Dracula's three wives close in on him. Dracula waves them away, then attacks Renfield himself.
Aboard the schooner Vesta, Renfield is a raving lunatic slave to Dracula, who hides in a coffin and feeds on the ship's crew. When the ship reaches England, Renfield is discovered to be the only living person. Renfield is sent to Dr. Seward's sanatorium adjoining Carfax Abbey.
At a London theatre, Dracula meets Seward (Herbert Bunston). Seward introduces his daughter Mina (Helen Chandler), her fiancÃ© John Harker (David Manners) and the family friend Lucy Weston (Frances Dade). Lucy is fascinated by Count Dracula. That night, Dracula enters her room and feasts on her blood while she sleeps. Lucy dies the next day after a string of transfusions.
Renfield is obsessed with eating flies and spiders. Professor Van Helsing (Edward Van Sloan) analyzes Renfield's blood and discovers his obsession. He starts talking about vampires, and that afternoon Renfield begs Seward to send him away, claiming his nightly cries may disturb Mina's dreams. When Dracula calls Renfield with wolf howling, Renfield is disturbed by Van Helsing showing him wolfsbane, which Van Helsing says is used for protection from vampires.
Dracula visits Mina, asleep in her bedroom, and bites her. The next evening, Dracula enters for a visit and Van Helsing and Harker notice that he does not have a reflection. When Van Helsing reveals this to Dracula, he smashes the mirror and leaves. Van Helsing deduces that Dracula is the vampire behind the recent tragedies.
Mina leaves her room and runs to Dracula in the garden, where he attacks her. She is found by the maid. Newspapers report that a woman in white is luring children from the park and biting them. Mina recognizes the lady as Lucy, risen as a vampire. Harker wants to take Mina to London for safety, but is convinced to leave Mina with Van Helsing. Van Helsing orders Nurse Briggs (Joan Standing) to take care of Mina when she sleeps, and not to remove the wreath of wolfsbane from her neck.
Renfield escapes from his cell and listens to the men discuss vampires. Before his attendant takes Renfield back to his cell, Renfield relates to them how Dracula convinced Renfield to allow him to enter the sanitorium by promising him thousands of rats with blood and life in them. Dracula enters the Seward parlour and talks with Van Helsing. Dracula states that Mina now belongs to him, and warns Van Helsing to return to his home country. Van Helsing swears to excavate Carfax Abbey and destroy Dracula. Dracula attempts to hypnotize Van Helsing, but the latter's resolve proves stronger. As Dracula lunges at Van Helsing, he withdraws a crucifix from his coat, forcing Dracula to retreat.
Harker visits Mina on a terrace, and she speaks of how much she loves "nights and fogs". A bat flies above them and squeaks to Mina. She then attacks Harker but Van Helsing and Seward save him. Mina confesses what Dracula has done to her, and tells Harker their love is finished.
Dracula hypnotizes Briggs into removing the wolfsbane from Mina's neck and opening the windows. Van Helsing and Harker see Renfield heading for Carfax Abbey. They see Dracula with Mina in the abbey. When Harker shouts to Mina, Dracula thinks Renfield has led them there and kills him. Dracula is hunted by Van Helsing and Harker knowing that Dracula is forced to sleep in his coffin during daylight, and the sun is rising. Van Helsing prepares a wooden stake while Harker searches for Mina. Van Helsing impales Dracula, killing him, and Mina returns to normal.

Question: What makes Renfield faint?
Answer: a bat appears