When using the standard Stardew Valley dialogue, every conversation the player has with a character increases their friendship points. With Midnight Stardew there is no impact anything by default. The effects that a conversation has on the character or the player must be set within the conversation.

{
    "Name": "Robin",
    "Conversations": 
    [
        {
            "Statement": [ "Nice to see to you!" ]
            "Effects":  
            { 
                "Hearts": "20" 
            }
        }
    ]          
}

The above conversation would have the same impact that a normal Stardew Valley dialogue would have, increasing the friendship points by 20 when the player speaks to the character. Note, there are 250 friendship points per heart in Stardew Valley.

You can also impact custom relationship stats between the player and the character.

{
    "Name": "Robin",
    "Conversations": 
    [
        {
            "Statement": [ "Nice to see to you!" ]
            "Effects":  
            {
                "Stats":
                {
                    "Rapport": "80"
                }
            }
        }
    ]          
}

The above file would add 80 points to the "Rapport" that the character feels towards the character. There is a difference between increasing the level of custom stats and the built in friendship points. As noted above, the player gets a heart with an NPC every 250 points. For custom Midnight Stardew stats the player gains a level every 1000 points. This change was made to make it easy to convert between points and levels.

{
    "Name": "Robin",
    "Conversations": 
    [
        {
            "Reqs": 
            {
                "Hearts": "3", // Requires 750 friendship points
                "Stats": 
                {
                    "Rapport": "3" // Requires 3000 Rapport points
                }
            }
            "Statement": [ "Nice to see to you!" ]
            "Effects":  
            {
                "Hearts": "20",
                "Stats":
                {
                    "Rapport": "80"
                }
            }
        }
    ]          
}

As illustrated above, if you want custom stats to gain levels at the same rate as the built in Stardew Valley hearts then you will need to multiply the Stardew Valley friendship points gained or lost by four.