Ronald Cooke, who suggested he did not believe Tina Billingham intended to kill herself, put a sword stick on the seat between them after she said:' If I had a knife now I would stab myself.'
The 55-year-old defendant, who was on a trip with her in his van when tragedy struck, continued in an interview read to Wolverhampton Crown Court: "I said 'the only knife in here is this one of your uncle.'
"I have got it and put it on the seat, not expecting what she actually did. I put it on the seat between us. I called her bluff, that is all I can say.
"It was a case of don't threaten me with doing yourself harm if things aren't going your own way. That's what she did in the past when she took an overdose of paracetamol. I don't think she has done it as a fatal act to herself."
Cooke, who had lived with Miss Billingham at a rented house in Granville Road, Old Hill for over 15 years, said he took threats she would harm herself 'with a pinch of salt,' alleging she did this in a bid to get her own way. But he added: "She had never said she was going to stab herself before."
When the interviewer asked what would have prompted her to do that, he replied: "I don't know. I am blank on that."
The defendant said he was driving at the time and a bag on the seat between the pair obscured his view, meaning he had not seen the stabbing.
He continued: "She was right next to it (the sword stick). I didn't see her do the movements. I did not realise she had done it.
"There was no blood on the knife, maybe because it was pulled out fast. I must have put it in the sheath and put it behind us. I said to her something like: 'See you ain't done anything.' In my mind she had made a threat and not gone through with it - but she had.
"I never had another word from her. She was just staring at me. I never noticed her injuries when she slumped forward just seconds later. She was not responding to me. She had actually used it. I knew she had."
Cooke drove Miss Billingham to the doctor's surgery at Rowley Healthcare in Hawes Lane, Rowley Regis where she was treated by staff for two knife wounds through her heart and liver until paramedics arrived to take her to hospital. She was certified dead soon afterwards.
The defendant pleads not guilty to murdering his partner on February 6 and the trial continues.