When you use character datatypes(char,varchar,nchar,etc),always make sure that column has enough length to have data. Otherwise sometimes you would get errors and sometimes you wont. Consider the following example

Declare @t table(i int, test_col varchar(2))
Insert into @t 
select 1,'test'
Select i,test_col from @t 

Well. You get the error

Msg 8152, Level 16, State 14, Line 4 String or binary data would be truncated.The statement has been terminated. 

Because the column length of test_col is not enough to have the value 'test' But only when you insert data to a table, you get this error. In other cases you dont

Declare @v varchar(2)
set @v='test'select @v

Now there is no error but the value is truncated and only first two characters are assigned to the variable. The same case is applied when you use a stored procedure with input parameter

create procedure test(@test_param varchar(2))as
Select @test_param
Go
EXEC test1 'test'
Go
drop procedure test

Now see what happens when you dont specify the length

Declare @v varchar
set @v='test'
select @v

By default the length is 1 Consider another example

Select cast('This has more than thirty characters' as varchar)

The result is This has more than thirty char When you cast a literal to character type without specifying the length, by default it is 30. So you need to specify the enough column length when you use character datatypes