Asked if Koenig might try to hurt himself, Walter Koenig replied, "I think it's almost impossible not to explore that as one of the options."
Then, with tears welling in his eyes, he added, "There are so many people out there who really care about him."
Koenig's parents went public with their concern for their son earlier this week when they appeared on "Good Morning America" and the "Today" show. Koenig's sister Danielle appeared on CBS' "The Early Show" and "Larry King Live."
Koenig was reported missing by his parents Feb. 18.
Vancouver police said he was visiting friends in West Vancouver when he disappeared Feb. 14. He did not show up for his return flight to California two days later.
That same day, Feb. 16, Walter Koenig received a letter from his son, which concerned him because of its "despondent tone."
"He's been depressed," he told ABC News. "He's trying to get ahead in this business and he's been working at it a long time."
Koenig largely dropped out of the spotlight after playing the role of the goofy sidekick to actor Kirk Cameron's Mike Seaver on "Growing Pains." After several smaller movie and television roles, he found work as a video producer and editor.
Lance Miccio, who has known Koenig for seven years and collaborated with him on multiple projects, said Koenig's depression was "something he dealt with."
"He faced it, didn't shirk it," Miccio told ABCNews.com. "He received treatment for it. It was a day-to-day thing that was woven into his fabric. It never affected his work. Maybe he was a little more grouchy than usual. But he was able to deal with it.
"I don't think anything happened," Miccio said. "There was no trauma. It was just something he dealt with."
Koenig's father concurred on his Web site, where he has been posting information about his missing son.
"I think it's something that has been a part of his makeup for a long time. There's no single trauma. There's no episode. There's nothing of that nature," he wrote.
He added "drugs were not an issue."
Miccio said Koenig wasn't married, but dated.
"He had an active social calendar," he said, "and was extremely close to his family."
But it appeared that his family and friends were not aware until after he left for Vancouver that he may not have planned to return to California.
Miccio said Koenig sold or gave away all of his possessions, gave 30-days notice to the landlord of the Venice, Calif., apartment that he rented for 14 years and told a mutual friend that he was going to Vancouver to "start over."
While Miccio was out of town, Koenig dropped off a bag of belongings and gifts at Miccio's door.
"He hooked it on my doorknob -- a plastic bag of video tapes and stuff we had worked together on. Even gifts I had given him," Miccio said. "He just returned everything with no note or anything."
Judy Koenig told reporters the last time she heard from her son was when he left a message on Feb. 8 or 9 asking how she was recuperating from recent surgery.
Longtime friend Jenny Magenta, who was the last to see Koenig in Vancouver, said he came to stay with her Feb. 10, after visiting mutual friends in Toronto. She last saw him on the evening of Feb. 14.
"That night I went to bed and in the morning he was gone," Magenta said.
She said Koenig left a note that said simply, "Thank you, Andrew."
ABC News' Jason Stine contributed to this report.