To develop their relationship realistically, start with their initial meeting. If the male OC is from a different background, there should be some level of distrust or misunderstanding at first. For example, if he's from a poorer region and she's a Lannister, she may look down on him. As they interact more, small acts of kindness or shared interests can start to break down the barriers. For instance, if they both have a love for a certain type of horse, it can be a starting point. Their relationship should also be affected by the political climate around them. If Cersei's power is threatened, she may be more hesitant to show affection for the male OC for fear of looking weak.
First, consider their personalities. Cersei is a complex and often ruthless character. The male OC needs to have qualities that can either complement or challenge her. If he's too meek, she'll likely dismiss him. If he's too aggressive, she may see him as a threat. Their relationship should develop gradually over time. Maybe they start as reluctant allies in a political scheme. As they work together, they begin to understand each other's motives and desires. Also, the power dynamics play a huge role. Cersei is used to being in control, so if the male OC starts to gain influence over her, it should be a slow and believable process. Their relationship should also be influenced by the events in the world around them, like wars or political upheavals.