Stage Fright

Eve Gill (Jane Wyman) is an aspiring actress at RADA. She is interrupted in rehearsal by her friend (and crush), actor Jonathan Cooper (Richard Todd), the secret lover of flamboyant stage actress/singer, Charlotte Inwood (Marlene Dietrich). Via a flashback he says Charlotte visited him after killing her husband; she was wearing a bloodstained dress. Jonathan claims he went back to her house for another dress, but was seen by Charlotte's cockney dresser, Nellie Goode (Kay Walsh). He escaped the police and needs help.
Eve takes him to her father's house on the coast to hide. Commodore Gill (Alastair Sim) notices that the blood on Charlotte's dress has been smeared on deliberately; he and Eve think that Jonathan was framed by Charlotte. Jonathan angrily destroys the dress and thus the most useful piece of evidence.
Eve starts to investigate. She hears Charlotte's dresser Nellie Goode boasting about the notoriety in a bar. While she is there, Eve meets Detective Inspector Wilfred O. Smith (Michael Wilding), and they become friendly. Eve then poses as a reporter; she bribes Nellie to tell Charlotte she is ill, and to introduce her cousin, "Doris Tinsdale," as a replacement. Using her acting skills, Eve becomes "Doris" and starts working for Charlotte. Eve discovers Charlotte is having an affair with her manager Freddie Williams (Hector MacGregor).
Eve and "Ordinary" Smith become more friendly. When Smith visits Charlotte, Eve has to disguise the fact that she is also "Doris" the maid. Smith makes a courtship visit to Eve and her mother at home, where the Commodore drops subtle hints that Jonathan has left the seaside house.
Despite her widowed status, Charlotte continues to perform her West End musical show. Jonathan comes to her dressing room asking her to accompany him abroad. She casually tells him no, but he says he still has the bloodstained dress. The police search for Jonathan, and Eve again helps him escape. He hides out at the Gill's London residence. He is grateful to Eve, but she is starting to fall in love with Detective Smith.
Smith and Eve kiss in a taxi on the way to the RADA garden party, where Nellie Goode confronts Eve, demanding more blackmail money. Eve does not have enough, so Eve's father comes to give Nellie more cash. Freddie Williams spots Eve (thinking she is "Doris") and orders her to help Charlotte, who is to sing on stage in a tent. During the performance, Commodore Gill gets a small boy to carry a doll wearing a bloodstained dress up onto the stage. Charlotte collapses and "Doris" has to help.
Seeing this, Smith confronts Eve and the Commodore, but Eve proclaims her true affection for Smith as well as Jonathan's innocence. They persuade Smith to set Charlotte up. Once the theatre has closed, they use a hidden microphone and "Doris" tells Charlotte she has the bloodstained dress. Smith and his men listen using the theatre loudspeakers. Charlotte admits planning her husband's death, but says that Jonathan actually committed the murder. Charlotte offers Eve 10,000 pounds to keep quiet.
Eve sees that Jonathan has been brought to the theatre by the police, but he escapes. Charlotte realizes her conversation with Eve was broadcast to the detectives, and that she will be charged as an accessory to murder. Detective Smith tells the Commodore that Jonathan really did kill Mr. Inwood, and that Jonathan has killed before, though he got off on a plea of self-defense.
Hiding below stage, Jonathan confesses to Eve that Charlotte goaded him into killing her husband. His flashback story was all lies, and he was the one who smeared more blood onto the dress. He threatens to kill Eve to give a reason for pleading insanity, but she escapes, and in the confusion, Jonathan is killed by the stage's falling safety curtain.

Question: Who is Charlotte Inwood's secret lover?
Answer: Johnathan Cooper