Mock only one method of instace.

public static int ExecuteCommandWithReturnValue(string storedProcedure, ref SqlParameter[] arParams, int commandTimeout, string connectionString)
{
  int returnValue = 0;
   SqlConnection connection = new SqlConnection(connectionString);
   SqlCommand command = new SqlCommand(storedProcedure, connection);
   command.CommandType = CommandType.StoredProcedure;
   command.CommandTimeout = commandTimeout;
      foreach (SqlParameter parameter in arParams)
            {
                command.Parameters.Add(parameter.ParameterName,  parameter.SqlDbType);
                command.Parameters[parameter.ParameterName].Value = parameter.Value;
                command.Parameters[parameter.ParameterName].Direction = parameter.Direction;
            }
   connection.Open();
   command.ExecuteNonQuery();
   returnValue = (int) command.Parameters[0].Value;
   connection.Close();
   return returnValue;
}

SqlParameter[] arParams = new SqlParameter[] { new SqlParameter("par1", "val1") };
arParams[0].Direction = ParameterDirection.Input;
arParams[0].DbType = DbType.String;
            using (RecordExpectations r = RecorderManager.StartRecording())
            {
                SqlConnection connection = new SqlConnection(null);

                connection.Open();
                //command.ExecuteNonQuery();   how to mock only this method
                //r.Return(2);
                connection.Close();
            }
            Assert.IsInstanceOfType(typeof(int), Common.ExecuteCommandWithReturnValue("storedProcedure", ref arParams, 2, "connectionString"));

SqlCommand command = new SqlCommand(null,null);

command.CommandType = CommandType.StoredProcedure;
command.CommandTimeout = 2;
command.Parameters.Add("par1", SqlDbType.VarChar);
r.Return(new SqlParameter());
command.Parameters["par1"].Value = "val1";
command.Parameters["par1"].Direction = ParameterDirection.Input;