C# Interface vs. Abstract best practice question

I'm really just trying to make sure I'm not misunderstanding interfaces and how they are implemented.  Here is some code to show what I am doing:
This is my interface:

namespace InterfaceTest
{
    public interface IComment
    {
        Guid ID { get; }
        string Comment { get; set; }
        string EnteredBy { get; set; }
        DateTime UtcDateTime { get; }

        bool Insert();
    }
}

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This is my implementation of the interface:

namespace InterfaceTest
{
    public class CustomerComment : IComment
    {
        public string Comment { get; set; }
        public string EnteredBy { get; set; }
        public Guid ID { get; private set; }
        public Guid CustomerID { get; private set; }
        public DateTime UtcDateTime { get; private set; }
        public CustomerComment(string comment, Guid customerID, string enteredBy)
        {
            this.Comment = comment;
            this.CustomerID = customerID;
            this.EnteredBy = enteredBy;
        }

        public static List<CustomerComment> GetComments()
        {
            List<CustomerComment> result = new List<CustomerComment>();

            result.Add(new CustomerComment("comment",Guid.Empty,"username"));

            return result;
        }

        public bool Insert()
        {
            bool result = false;

            return result;
        }
    }
}

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My questions pertain to the CustomerID Property and Static GetComments method I added.  Should this not be done since it goes beyond the interface definition?  And if it shouldn't be done then what is the way I should do it?  For instance I would be Implementing another comment for say UserComment : IComment where there wouldn't be a CustomerID but instead a UserID.