SQL 2012 :: Calculate Time Difference For Consecutive Rows?

I have a table like this.

CREATE TABLE Table1
([S_ID] varchar(7), [S_ACTV_CODE] varchar(4), [S_USER] varchar(5), [S_DATETIME] varchar(19), [S_ACT_IND] int)
;
INSERT INTO Table1
([S_ID], [S_ACTV_CODE], [S_USER], [S_DATETIME], [S_ACT_IND])
VALUES
('AAA-111', NULL, 'USER1', '2015-06-15 00:21:06', 0),
('AAA-111', '2', 'USER1', '2015-06-15 00:21:07', 0),

[code]....

Basically I want to calculate the time spent by S_Users on a particular S_ACTV_CODE:

- S_ACTV_CODE_PREV means the previous active records.

- S_START_TIME is the time of S_DATETIME when a S_ACTV_CODE starts

- S_END_TIME is the time before a S_ACTV_CODE changes to another S_ACTV_CODE

- For the first record, S_ACTV_CODE is null, so there is no S_ACTV_CODE_PREV, so S_ACTV_CODE_PREV is NULL

- For the second record S_ACTV_CODE has some value, but S_ACTV_CODE_PREV is NULL for first record. So second record S_ACTV_CODE_PREV is also NULL

- For the last record (means S_ACTV_IND = 1), the user is currently working on it and S_ACTV_CODE is not changed. So S_END_TIME is a open time and we want to keep it as NULL

So the result should be as below:

S_ID S_ACTV_CODE_PREV S_ACTV_CODE_CURR S_USER S_START_TIME
S_END_TIME TIME_SPENT (in Sec)
AAA-111 NULL NULL USER1 2015-06-15 00:21:06
2015-06-15 00:21:07 1
AAA-111 NULL 2 USER1 2015-06-15 00:21:07
2015-06-17 03:20:33 183566
AAA-111 2 4 USER2 2015-06-17 03:20:33

[code]....