public class MyFixture
{
    private readonly ICloseLogFileCommand _closeCommand = Substitute.For<ICloseLogFileCommand>();

    private readonly ILog _log = Substitute.For<ILog>();

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