What if I kissed this stranger? What if I just stopped turning up for work?

While intrusive thoughts like these can pop into your mind, they are common and usually easy to brush off for what they are — unwanted, random and meaningless.

But how much a person can dismiss intrusive thoughts varies. And when those thoughts stick around, they can cause distress and anxiety, and be symptomatic of obsessive-compulsive disorder (OCD).

Brisbane-based writer and filmmaker Martin Ingle knows this well. He was diagnosed with OCD in his early 20s.

His intrusive thoughts and compulsions didn't fit the stereotype of the obsessively cleaning type of OCD often represented in pop culture.

Martin was instead plagued by concerns about his sexuality, or hurting people.

"I would not leave the house. I would avoid members of my family. I lost a lot of work because I didn't want to do work around certain people. I retreated from almost all aspects of my life," he told ABC Radio National's All In The Mind.

When intrusive thoughts get stuck in your head

Why we have intrusive thoughts is a bit of a mystery, according to Vlasios Brakoulias, a psychiatrist and professor at Westmead Hospital and the University of Sydney.

We don't know their exact cause, but the mechanisms at play may be similar to what happens when we dream.

"Just like dreams, these are subconscious thoughts, and they tend to reflect some of the instinctual feelings that people have," Professor Brakoulias says.

"They might be embarrassing. They might be against what we really want to do."

And if someone is experiencing poor mental health, it can be difficult to let intrusive thoughts go.

For example, if you're stressed or anxious, you may find it harder to determine what thoughts are intrusive and what are conscious, Professor Brakoulias says. 

Intrusive thoughts can be especially challenging for people with OCD. Thoughts become recurring and inescapable. Instead of being dismissed, they lead the person to wondering, "Why did I think of that? What if it's true?"

This rumination can lead to developing compulsions, repetitive physical or mental actions to combat the persistent intrusive thoughts. This might look like excessive cleaning, double-checking, or avoiding certain situations altogether.

Naming thoughts as intrusive and random may make them easier to dismiss. (Getty Images: Eoneren)

To be diagnosed with OCD, a person's obsessive thinking and compulsions need to take up at least an hour every day, Professor Brakoulias says.

"So people with the disorder are unable to function because of the severity of their obsessions and compulsions."