public class MyFixture
{
    private readonly ICloseLogFileCommand _closeCommand = A.Fake<ICloseLogFileCommand>();

    private readonly ILog _log = A.Fake<ILog>();

    public MyFixture()
    {
        A.CallTo(() => _log.Name).Returns(@"C:\Path\To\File.log");
        A.CallTo(() => _log.Count).Returns(42);
        A.CallTo(() => _log.Offset).Returns(TimeSpan.FromSeconds(69));
    }
    ...
}