Problem with sp_executesql

I try to write query that use sp_executesql to query data by Like operation with 1 parameter like below:
execute sp_executesql N'SELECT DISTINCT au_id,
au_lname,au_fname
FROM authors
WHERE au_lname LIKE @au_lname
',
N'@au_lname nVarChar',
@au_lname = N'%Cas%'

but It return all rows regardless of changing condition to any value.

But if i don't use sp_executesql like below:

SELECT DISTINCT au_id,
au_lname,au_fname
FROM authors
WHERE au_lname LIKE N'%Cas%'

It's correct!

Can anyone tell me why?

Thanks

Is this alternative approach acceptable to you? I know you miss out on the pre-compile performance advantage...

Declare @SQLString nVarChar(100), @au_lname VarChar(20)
Set @au_lname = 'Cas'
Set @SQLString = 'SELECT DISTINCT au_id,
au_lname,au_fname
FROM authors
WHERE au_lname LIKE ''%' + @au_lname + '%'''
execute sp_executesql @SQLString

Declaring a variable as nvarchar means that it will have a max length of 1. You can figure the rest.

Why dynamic sql at all?