[DataMember]
public virtual Role Role { get; set; }

[DataContract]
public class Role
{
    [DataMember]
    public virtual string Name { get; set; }
    [DataMember]
    public virtual string Description { get; set; }
}

    [DataContract]
[KnownType(typeof(Role))]
public class User 
{
    [DataMember]
    public virtual int UserId { get; set; }
    [DataMember]
    public virtual string UserName { get; set; }
    [DataMember]
    public virtual string FirstName { get; set; }
    [DataMember]
    public virtual string LastName { get; set; }
    [DataMember]
    public virtual string Email { get; set; }
    [DataMember]
    public virtual string Company { get; set; }
    [DataMember]
    public virtual string Comments { get; set; }
    [DataMember]
    public virtual bool IsApproved { get; set; }
    [DataMember]
    public virtual bool IsLocked { get; set; }
    [DataMember]
    public virtual bool LoggedIn { get; set; }

}

    [OperationContract]
    [WebInvoke(Method = "POST",
    ResponseFormat = WebMessageFormat.Json,
    RequestFormat = WebMessageFormat.Json,
    BodyStyle = WebMessageBodyStyle.Wrapped
    )]
User GetUserDetails(string userName);

<serviceDebug httpHelpPageEnabled="true" includeExceptionDetailInFaults="true"/>